Myopathies are disorders that result in functional impairment of muscles. Muscular dystrophy (MD) refers to genetic diseases that are characterized by progressive weakness and degeneration of skeletal muscles. Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are the most common childhood forms of muscular dystrophy. They are recessive disorders and because the gene responsible for DMD and BMD resides on the X-chromosome, mutations mainly affect males with an incidence of about 1 in 3500 boys.
DMD and BMD are caused by genetic defects in the DMD gene encoding dystrophin, a muscle protein that is required for interactions between the cytoskeleton and the extracellular matrix to maintain muscle fiber stability during contraction. DMD is a severe, lethal neuromuscular disorder resulting in a dependency on wheelchair support before the age of 12 and DMD patients often die before the age of thirty due to respiratory- or heart failure. In contrast, BMD patients often remain ambulatory until later in life, and have near normal life expectancies. DMD mutations in the DMD gene are characterized by frame shifting insertions or deletions or nonsense point mutations, resulting in the absence of functional dystrophin. BMD mutations in general keep the reading frame intact, allowing synthesis of a partly functional dystrophin.
During the last decade, specific modification of splicing in order to restore the disrupted reading frame of the dystrophin transcript has emerged as a promising therapy for Duchenne muscular dystrophy (DMD) (van Ommen, van Deutekom, Aartsma-Rus, Curr Opin Mol Ther. 2008; 10(2):140-9, Yokota, Duddy, Partidge, Acta Myol. 2007; 26(3):179-84, van Deutekom et al., N Engl J Med. 2007; 357(26):2677-86).
Using antisense oligonucleotides (AONs) interfering with splicing signals the skipping of specific exons can be induced in the DMD pre-mRNA, thus restoring the open reading frame and converting the severe DMD into a milder BMD phenotype (van Deutekom et al. Hum Mol Genet. 2001; 10: 1547-54; Aartsma-Rus et al., Hum Mol Genet 2003; 12(8):907-14). In vivo proof-of-concept was first obtained in the mdx mouse model, which is dystrophin-deficient due to a nonsense mutation in exon 23. Intramuscular and intravenous injections of AONs targeting the mutated exon 23 restored dystrophin expression for at least three months (Lu et al. Nat Med. 2003; 8: 1009-14; Lu et al., Proc Natl Acad Sci USA. 2005; 102(1):198-203). This was accompanied by restoration of dystrophin-associated proteins at the fiber membrane as well as functional improvement of the treated muscle. In vivo skipping of human exons has also been achieved in the hDMD mouse model, which contains a complete copy of the human DMD gene integrated in chromosome 5 of the mouse (Bremmer-Bout et al. Molecular Therapy. 2004; 10: 232-40; 't Hoen et al. J Biol Chem. 2008; 283: 5899-907).
Recently, a first-in-man study was successfully completed where an AON inducing the skipping of exon 51 was injected into a small area of the tibialis anterior muscle of four DMD patients. Novel dystrophin expression was observed in the majority of muscle fibers in all four patients treated, and the AON was safe and well tolerated (van Deutekom et al. N Engl J Med. 2007; 357: 2677-86).